Effect of temperature on the biological parameters of pink bollworm, Pectinophora gossypiella Saunders (Lepidoptera: Gelechiidae)

Pink bollworm (PBW) Pectinophora gossypiella is an important pest cotton worldwide. There are multiple factors which determines the occurrence and distribution of P. gossypiella across different cotton growing regions of the world, and one such key factor is ‘temperature’. The aim was to analyze the life history traits of PBW across varying temperature conditions. We systematically explored the biological and demographic parameters of P. gossypiella at five distinct temperatures; 20, 25, 30, 35 and 40 ± 1 °C maintaining a photoperiod of LD 16:8 h. The results revealed that the total developmental period of PBW shortens with rising temperatures, and the highest larval survival rates were observed between 30 °C and 35 °C, reaching 86.66% and 80.67%, respectively. Moreover, significant impacts were observed as the pupal weight, percent mating success, and fecundity exhibited higher values at 30 °C and 35 °C. Conversely, percent egg hatching, larval survival, and adult emergence were notably lower at 20 °C and 40 °C, respectively. Adult longevity decreased with rising temperatures, with females outliving males across all treatments. Notably, thermal stress had a persistent effect on the F1 generation, significantly affecting immature stages (egg and larvae), while its impact on reproductive potential was minimal. These findings offer valuable insights for predicting the population dynamics of P. gossypiella at the field level and developing climate-resilient management strategies in cotton.

www.nature.com/scientificreports/ The effect of climate change on the agriculture sector is a major concern.Global temperature is expected to increase by 1.5-4.5 °C by the end of the century 12 .The physiological response of species to abiotic factors such as temperature, rainfall, humidity and photo-period has gained considerable attention in the scientific community for more than a century.Of these, temperature is one of the important variables of the environment that can influence phenotypic plasticity and affect various aspects of the life history parameters of insects 13 .The distribution and abundance of pests in an ecosystem are largely influenced by temperature 14 .Insects may undergo biophysical changes like the accumulation of sugars and varying the fluidity of cellular membranes in accordance with temperature 15 .The increased metabolic rate of an organisms corresponding with increased temperature subsequently results in a higher growth rate with a short developmental time 16 .However, the rate of changes in development of insects occurs at specific temperature range; even a small variation makes significant changes in the duration of the life cycle and ultimately reflects in its fitness 17 .
The life history parameters of insects, including developmental time, fecundity and longevity are of great importance for their survival 18 .Temperature greatly influences the duration of each instar of larvae, resulting in physiological changes in the development of gonads before reaching the adult stage.In addition, faster development can be more advantageous for an insect to escape from predation or parasitism 19 .In recent years, the severe infestation of P. gossypiella was observed in the central and northern parts of India; the outbreak of this pest was affected by changes in climatic factors.However, the information regarding thermal effects on the life history traits of P. gossypiella and its transgenerational effect is minimal.In this context, it is crucial to investigate the effect of temperature on the life parameters of P. gossypiella under the current changing climatic conditions.In the present study, we systematically investigated the developmental and reproductive traits under six different temperatures to evaluate the thermal fitness of P. gossypiella.

Developmental time and survival rate
The developmental stages of P. gossypiella from egg to pupa were significantly influenced under different thermal regimes (Table 1).The incubation period decreased substantially when exposed to a temperature of 20 to 40 o C (F = 44.83,p < 0.001).Similarly, the larval developmental time decreased as temperature increased at each instar (F = 29.51,66.81, 60.37, 69.38, p < 0.001), and the total duration of larvae also differed significantly at respective temperatures.The developmental time of egg, larvae and pupae were significantly longer at 20 o C and shorter at 40 o C; whereas, no significant difference was observed between 30 and 35 o C (Table 1 and 3).Egg hatching and larval survival rate was lower at 20 and 40 o C; but, they did not differ significantly at 25, 30 and 35 o C (Fig. 1).The similar trend was also shown in the age-stage-specific survival rate (S xj ) and age-stage life expectancy (e xj ).Further, the duration of development of pupae varies from 7 to 11 days at different temperatures.Interestingly 13.33% (20 individuals) did not pupate at 20 o C and underwent diapause at fourth instar; the total diapause period was ranged from 43 to 105 days, respectively.

Pupal weight and adult emergence
The pupal weight of both male (F = 13.06,p < 0.001) and female (F = 16.93,p < 0.001) varied significantly at different temperatures and it was highest at 30 o C (21.62 ± 0.66 and 25.42 ± 0.80 mg) and 35 o C (21.97 ± 0.70 and 25.40 ± 0.85 mg), respectively and the lowest pupal weight was recorded at 20 o C (16.70 ± 0.45 and 19.64 ± 0.72 mg) (Fig. 2).However, the pupal weight of females was comparatively higher than that of males in all treatments (p < 0.001).The regression coefficient of male and female pupal weight (r = 0.32, 0.50) showed a negative relation with temperature.Further, the percent adult emergence of P. gossypiella was also significantly affected at different temperature; the maximum adults emerged between 25 to 35 o C and lower emergence was recorded at 20 and 40 o C (Fig. 1).

Mating success, fecundity, sex ratio and adult longevity
The percentage of mating success of P. gossypiella showed significant differences and it was highest at 30 o C (84 ± 6.63), followed by 35 o C (80 ± 5.16).The individuals were exposed to 20 o C exhibited less mating success (48 ± 6.16).The fecundity of P. gossypiella also differed significantly at respective temperatures (F = 30.83,p < 0.001).The highest fecundity was recorded at 30 and 35° C with a mean of 77.92 ± 3.35 and 80.68 ± 2.81 eggs/ female, respectively, and lowest fecundity at 20 o C (52.48 ± 2.88) (Figs.3-7 and Table 2).However, no significant differences were recorded in the sex ratio at all temperatures (p < 0.804).The longevity of male and female adults (F = 15.48 and 9.17, p < 0.001) was significant, and it was varied from 9.92 ± 0.62 to 13.08 ± 1.49 in males and 11.36 ± 1.37 to 14.72 ± 1.33 in female adults, respectively.As the temperature increased, the longevity of both male and female adults were decreased.However, the longevity of females was higher than that of male adults in all the treatments.Furthermore, the intrinsic rates of increase (r), finite rates of increase (λ), net reproductive rates (R 0 ), gross reproduction rate (GRR), mean generation times (T) and doubling time (DT) for various groups are shown in Table 3.

Thermal effect on F 1 generation
The persistence effect of thermal stress over the F 1 generation of P. gossypiella on various biological parameters was recorded (Table 4).Similar to F 0 , the egg (F = 18.64, p > 0.001), and larval period (F = 10.65,p > 0.001) showed significant differences and the total duration decreased as temperature increased.However, no significant differences were recorded in egg hatching percentage and pupal weight, but it was slightly higher from the female parent exposed at 30 o C (79.44 ± 3.03, 22.30 ± 0.78) than others.In addition, the percentage of successful mating and fecundity was comparatively higher from those reared at higher temperatures viz., 30, 35 and 40 o C. Further, significant differences were also recorded on the longevity of male (F = 4.05, p > 0.01) and females (F = 3.67, p > 0.01) adults.The longevity was comparatively higher at 20 o C and female adults lived longer than males.

Discussion
Pectinophora gossypiella is a multivoltine, mostly stenophogous herbivore that feeds on cotton and other closely related species.The climate change has shifted the distribution of temperature variability and extremities which may impact the natural development of an organism and successful colonization 20 .In this context, understanding the life history traits of P. gossypiella is imperative to develop climate resilient management strategies.In the present study, P. gossypiella possesses the traits to become a successful colonizer species at varied temperatures.The mean duration of egg, larval and pupal development was substantially longer at 20 o C and shorter at 40 o C.However, the results from other studies were slightly inconsistent with our observation and reported that the duration of egg development was close to 10 days at 20 o C and 4 days at 35 o C, respectively 21,22 .Consequently, P. gossypiella took less time to complete a generation from egg to adult at 40°C with 26.09 ± 3.24 days compared to 42.74 ± 3.12 days at 20 °C, respectively.Hence, P. gossypiella can complete more generations in a season/year under high thermal regimes.However, this is mainly associated with metabolic reactions; many researchers reported that, an increase in temperature could hasten the process of metabolism at each level and produce more generations; therefore, the temperature is one of the important lifespan determinants in arthropods [23][24][25][26] .Furthermore, the percent egg hatchability and larval survival reached up to 81.2 and 86.7% at 30 °C; whereas, it was comparatively lower at 20 and 40 °C, indicating that any variation (low or high) from the normal range affects the developmental process; these results are consistent with some studies and they reported that, larval mortality of Spodoptera frugiperda (J.E.Smith) and P. gossypiella to be higher at 18 and 37 °C27,28 .Similarly, based on the simulation model the temperature threshold and thermal requirement for P. gossypiella under field conditions was varied between 25 to 30 °C22 .
In most cases, pupal weight determines the success of adult emergence, fecundity and adult longevity in holometabolous insects.In our study, the pupal weight was higher at 30 and 35 o C; and a corresponding increase in adult emergence, mating success and fecundity was recorded.Our study supports the 'fecundity advantage hypothesis' (i.e., the larger female produces more progenies) by producing more eggs from the females which were emerged from high pupal weight.Some studies reported that, the fecundity and fertility of P. gossypiella were significantly increased from those moths emerged from larger pupae; hence, pupal weight is a critical biological indicator of reproductive fitness in insects 29,30 .However, we found that the length of the pre-ovipositional and ovipositional periods decreases with increasing temperature.In addition, P. gossypiella exhibited excellent mating success up to 84 and 80% at 30 and 35 °C respectively; these results speculate that, adults may engage in copulation at the optimum temperature threshold and contribute more progenies for the next generation.Further, adult's longevity decreased as temperature increased in both male and females with a difference of 3.16 and 3.36 days from 20 to 40 °C, respectively; however, females lived longer than males in all treatments.This may be associated with 'heat flow'; where, a high rate of heat flow increases the rate of metabolism and reduces the lifespan; hence, metabolic rate is a crucial factor in determining longevity 31,32 .Further, the climatic variables such as precipitation, carbon-di-oxide, relative humidity together with temperature interact with plants in numerous ways and affects population dynamics of insect pests; as a result, expansion of host suitability and occurrence of new insect or invasive pests on various crops have been recorded over past century 33 .
The transgenerational effect of thermal stress on the biological parameters has been studied in some important lepidopteran insects like S. litura, S. frugiperda, Helicoverpa armigera and others [34][35][36] .However, no information is available on P. gossypiella, in this regard.Our study showed that, thermal stress on F 1 generation was evident and varied with different stages.The developmental duration of larval and pupal decreased with an increase in temperature; whereas, no significant differences were recorded in pupal weight and other reproductive parameters.These results suggest that, the impact of thermal stress was significant on the immature stages, but relatively less on the pupae and adults in F 1 generation as compared to their parent (F 0 ).It indicates that, progenies of P. gossypiella can overcome their parental thermal stress gradually, as it progresses to next developmental stage.Our data was consistent with other studies on other species like Frankliniella occidentalis, Drosophila serrata, and Aphidius ervi, which showed that, the impact of thermal stress on F 1 generation was evident at younger stage and lower at the matured stage [37][38][39] .whereas, a study on S. frugiperda reported that, the persistent effect can be observed up to F 2 generation 36 .Cell damage was evident when Saccharomyces cerevisiae was exposed to higher Vol:.(1234567890 www.nature.com/scientificreports/temperature, resulting reduction in ethanol production 41 .Therefore, insects can alleviate the thermal stress and steadily recover over generations by compromising some developmental traits 39 .In conclusion, Pectinophora gossypiella emerges as a significant cotton pest with a notable ability to establish itself in diverse environments.Our study underscores the pivotal role of temperature in shaping the developmental and biological aspects of P. gossypiella across various temperature regimes.Optimal conditions for egg and larval development, as well as overall reproductive potential, were identified within the temperature range of 30 to 35 °C.Deviations from this range led to reduced egg hatchability and increased larval mortality.Additionally, the study highlighted the enduring impact of thermal stress on the F1 generation, with observable recovery as they progressed to subsequent stages.These insights hold promise for predicting field-level population dynamics and devising resilient management strategies to ensure sustainable cotton production in the face of climate challenges.

Pink bollworm culture
Pectinophora gossypiella infested bolls were collected from different cotton fields in the region of Raichur (16°12′ 2.9″ N and 77° 21′ 44″ E), Karnataka, India, in 2020.However, samples were collected with the appropriate permission from the authority and it comply with relevant institutional guidelines.The collected bolls were dissected to extract the third and fourth instar larvae of PBW.The larvae were collected into the rearing boxes (30 cm diameter × 45 cm height) containing fresh cotton bolls until pupation under laboratory conditions (27 ± 2 o C, 55 ± 5% RH).The rearing boxes were checked daily and offered with fresh bolls as and when required.After pupation, males and females were sexed (based on gonad and anal pore) 40 and transferred into plastic containers.On the day of adult eclosion, a pair of adults were transferred into mating chambers (30 × 45 cm) for oviposition and covered with muslin cloth.Moistened cotton with a 10% honey solution was provided as adult food, and fresh cotton twigs were offered to facilitate egg laying.Oviposited twigs were collected daily and replaced with new twigs.Further, the eggs were observed until hatching.

Experimental design
After hatching, the neonates were transferred to rearing trays (Tarson, 45 × 30 cm, 96 wells made of polypropylene) containing an artificial diet and covered with a thin plastic lid.The diet was prepared as per the protocol 30,42 .Each tray consists of 30 larvae, and the trays were placed in six illuminated BOD chambers (Micro technologies) with constant temperatures of 20, 25, 30, 35, and 40 ± 1 o C, with 65 ± 5% relative humidity and a photoperiod of 16:8 (16 h light: 8 h dark).The diet was changed and offered with fresh diet regularly till pupation.Pupae were sexed and placed in small plastic containers (10 × 15 cm) until adult emergence.Larvae from each tray were considered as replicates, and four replications were maintained in respective temperature conditions.
The life history traits were recorded from all individuals, which include the developmental time of egg, larvae and pupae.The survival rate of larvae, pupae, and pupal weight were counted at each temperature.After the emergence of adults, 30 pairs of moths were selected from each treatment and released a pair moths into plastic mating chambers (30 × 45 cm) separately, provided with cotton balls moistened in 10% honey as a food supplement and a fresh cotton twigs as ovipositional substrate.Cotton twigs were checked daily to record the egg count and replaced with fresh twigs whenever necessary.Those pairs did not lay eggs even 7 days after release was considered as unsuccessful mating.Further, the following observations were recorded, including length of pre-oviposition and oviposition periods, percent mating success, fecundity, and longevity of adults.

Transgenerational effect
The carryover effect of temperature over F 1 generation was investigated under laboratory conditions at 26 ± 2 o C, 65% RH, and photoperiod of 16:8 h light: dark cycle.About 200 eggs from each treatment in the fecundity experiment (F 0 ) were selected and the observations were recorded on developmental parameters including hatching percentage, developmental period of egg, larvae, and pupae.Each batch of larvae was reared independently on an artificial diet, as discussed above.After adult emergence, mating studies were conducted, as discussed above, with 30 pairs from each treatment.Further, reproductive parameters such as mating success (%), fecundity, and adult longevity were recorded individually.

Statistical analysis
The observed data for all individuals at different temperatures were analysed and the life table was constructed by using 'TWOSEX-MS Chart' software 43,44 .According to the age-stage, two-sex life table, the following parameters viz., Age-stage-specific survival rates (S xj ) = The means, standard errors and variances of the population parameters were estimated with 100,000 bootstrap replicates.Sigma plot 14.5 was used to create graphs.One-way ANOVA was used for analyse the thermal stress on F 0 and F 1 generation parameters such as developmental period of egg, larvae and pupae, pupal weight, pre-oviposition and oviposition periods, fecundity and longevity of adults using R software (Version 3.4.4) 45 n xj n 01 ; Age-specific survival rate (l x ) = m j=1 S xj ; Age-stage-specific fecundity (f xj ); Age-specific fecundity (m x ) = m j=1 S xj f xj m j=1 S xj ; Age-specific maternity (l x *m x ); Age-stage-specific life expectancy (e xj ) = m j=1 m j=1 S ij ; Age-stage-specific reproductive value (V xj ) = e −r(x+1) S xj n i=x e −r(i+1) m j=y S ij f ij ; Intrinsic rate of increase (r)-∞ x=0 e −r(x+1) l x m x = 1 ; Finite rate of increase (λ) = e r ; Net reproductive rate (R 0 ) = ∞ x=0 l x m x ; Mean generation time (T)= l n R 0 r were studied.

Figure 1 .Figure 2 .
Figure 1.The percent egg hatchability, larval survival and adult emergence of P. gossypiella at five different temperature.

Table 1 .
. Mean duration of (± SE) each developmental stage of Pectinophora gossypiella under different temperatures.Means (± SE) within one row followed by different letters are significantly different at the 0.01 level based on one-way ANOVA and Tukey's HSD multiple tests.

Table 2 .
Adult longevity and reproductive parameters of Pectinophora gossypiella under different temperatures.Values (± SE) within one row followed by different letters are significantly different at the 0.01 level based on one-way ANOVA and Tukey's HSD multiple tests.
Figure 3. Age-stage-specific survival rate (Sxj) of each developmental stage of P. gossypiella at different temperature.Figure 7. Reproductive value (Vx) of P. gossypiella at five different temperature.

Table 3 .
Population growth parameters of P. gossypiella at different temperatures.Means followed by different letters in the same row are significantly different by using paired bootstrap test based on the CI of difference.Standard errors were estimated by using 100,000 bootstrap resampling.

Table 4 .
Effect of thermal stress on life history traits of P. gossypiella over F1 generation.Values (± SE) within one row followed by different letters are significantly different at the 0.01 level based on one-way ANOVA and Tukey's HSD multiple tests.